Laminates of so-called FR-4 type, which result from layering and molding sheets of prepreg obtained by impregnating a resin component, such as an epoxy resin, into a glass cloth, are widely used as typical laminates that are utilized in printed wiring board for electronic devices. The denomination FR-4 is a category defined in the standards of the American NEMA (National Electrical Manufactures Association). Also known are composite laminates, referred to as CEM-3 types, having a configuration wherein a layer in which a nonwoven fabric is impregnated with a resin component is disposed as a core material layer, and layers in which glass cloth is impregnated with a resin component is layered, as surface layers, on both surfaces of the core material layer.
For instance, Patent Document 1 discloses a composite laminate having high bonding strength between layers, and being excellent in alkali resistance, heat resistance, and punching workability, the composite laminate being obtained through bonding of a resin-impregnated surface layer material, which is in turn obtained by impregnating a glass cloth with a resin varnish, onto both faces of a resin-impregnated material core material resulting from impregnating a nonwoven fabric and/or paper with a resin varnish. The resin varnish that is used in the core material contains a filler that combines talc and aluminum hydroxide, the blending ratio of talc and aluminum hydroxide ranges from 0.15 to 0.65:1, and the aluminum hydroxide is of boehmite type.
For instance, Patent Document 2 discloses, as a composite laminate that is thermally stable and exhibits excellent flame retardancy, a laminated material for printed circuit boards that is made up of a surface layer comprising a resin-impregnated glass woven fabric and an interlayer comprising a curable resin-impregnated glass nonwoven fabric, wherein the interlayer contains aluminum hydroxide of molecular formula Al2O3.nH2O (in the formula, n is a value >2.6 and <2.9), in an amount of 200 wt % to 275 wt % with respect to the resin in the interlayer.
Advances in the miniaturization of electronic devices in recent years have been accompanied with increasingly higher mounting densities of electronic components that are mounted on printed wiring boards. Further, mounted electronic components include now also LEDs (Light Emitting Diodes) or the like that demand heat dissipation properties and that are mounted in the form of a plurality of electronic components. Conventional laminates, as substrates that are used for such applications, are problematic on account of their insufficient heat dissipation properties. Further, reflow soldering is the dominant mounting method. In particular, reflow soldering that utilizes lead-free solder and that requires a high-temperature reflow process, has become a mainstream process, with a view to reducing environmental impact. High heat resistance is required in order to suppress, for instance, the occurrence of blisters in a reflow soldering process where such lead-free solder is utilized. Preserving mechanical workability, in terms of drilling by a drill and milling by a router, constitutes herein a further requirement. Sufficient flame retardancy so as to satisfy the V-0 level of UL-94, from the viewpoint of safety, is yet another requirement.
High thermal conductivity fillers are ordinarily packed at a high density in order to realize high thermal conductivity in substrate materials. However, high thermal conductivity fillers have advantages and disadvantages, in that, at present, they fail to satisfy all features that are required in printed wiring boards for use in LED illumination and in ECU boards that are used in engine rooms.
As is known, for instance, the thermal conductivity of a resin composition is enhanced by using alumina (aluminum oxide), which has high thermal conductivity. Alumina, however, is very hard, and impairs the mechanical workability of the resin composition.
The heat dissipation properties of a laminate improve if aluminum hydroxide, which has somewhat high thermal conductivity, is blended in with a view to imparting heat dissipation properties to the laminate. Flame retardancy is enhanced in this case as well. If aluminum hydroxide was excessive, however, problems arose in that the heat resistance of the laminate dropped significantly, and blisters were likelier to occur during solder reflow.
If aluminum hydroxide and alumina are used concomitantly and the content of alumina is substantial, bits may break during machining such as drilling by a drill and milling using a router, given the extremely high hardness of alumina. Therefore, problems arose in that bits had to be replaced frequently, and also in terms of poorer flame retardancy. A further problem arose in that sufficient heat resistance and thermal conductivity could not be achieved if the blending amount of aluminum oxide was reduced in order to curtail bit breakage.
Magnesium oxide has thermal conductivity comparable to that of alumina, and hardness lower than that of alumina, and affords good workability of a resin molded product into which magnesium oxide is blended. However, magnesium oxide characteristically changes into magnesium hydroxide as a result of moisture absorption. Conceivable measures against such an occurrence include special firing and surface processing, but the price involved is high, or particle size is large, or fluidity at high packing is poor, all of which are drawbacks.
Aluminum nitride and boron nitride have non-spherical shapes, and hence resin flow worsens on account of structural viscosity. Both aluminum nitride and boron nitride are also problematic as regards costs, given the very high price that they command.    Patent Document 1: Japanese Patent Application Publication No. S62-173245    Patent Document 2: Japanese Translation of PCT Application No. 2001-508002